Understanding Radioactive Decay
Atomic Structure
The Building Blocks of Matter
Everything around you, from the air you breathe to the screen you're reading this on, is made of atoms. For a long time, people thought atoms were the smallest possible things. But we now know they're made of even smaller, or subatomic, particles. Three key players make up the atom: protons, neutrons, and electrons.
Proton
noun
A subatomic particle with a positive electric charge, found within the nucleus of an atom.
Neutron
noun
A subatomic particle with no electric charge (it's neutral), found within the nucleus of an atom. It has a mass slightly greater than that of a proton.
Electron
noun
A subatomic particle with a negative electric charge that orbits the nucleus of an atom.
Protons and neutrons are the heavyweights, while electrons are incredibly light. Think of it this way: if a proton were a bowling ball, an electron would be a grain of sand. The number of protons defines what element an atom is. An atom with one proton is always hydrogen. An atom with six protons is always carbon.
The Atom's Core
The protons and neutrons are packed together in the center of the atom, forming what's called the nucleus. This nucleus is tiny and incredibly dense. The electrons, on the other hand, exist in a cloud of orbits around this nucleus, organized into layers called electron shells.
The positive charge of the protons in the nucleus is what keeps the negatively charged electrons in orbit, much like how the sun's gravity keeps Earth from flying off into space. In a neutral atom, the number of electrons is equal to the number of protons, so the positive and negative charges cancel each other out.
To keep track of the particles inside an atom, scientists use two important numbers: the atomic number and the mass number.
The atomic number (Z) is simply the number of protons in the nucleus. This number uniquely identifies an element.
The mass number (A) is the total number of protons and neutrons in the nucleus.
Variations on a Theme
While the number of protons for an element is fixed, the number of neutrons can change. Atoms of the same element that have different numbers of neutrons are called isotopes.
Isotope
noun
One of two or more forms of an element that have the same number of protons but a different number of neutrons in their nuclei.
For example, every carbon atom has 6 protons. But it can have 6, 7, or even 8 neutrons. This gives us different isotopes of carbon: carbon-12, carbon-13, and carbon-14. They are all still carbon, but they have slightly different masses because of the extra neutrons.
| Isotope | Protons (Z) | Neutrons (N) | Mass Number (A) |
|---|---|---|---|
| Carbon-12 | 6 | 6 | 12 |
| Carbon-13 | 6 | 7 | 13 |
| Carbon-14 | 6 | 8 | 14 |
Because they have the same number of protons and electrons, isotopes of an element have the same chemical properties. They react in the same way. However, the difference in neutron count can make some isotopes stable and others unstable, which is the origin of radioactivity.
Let's check your understanding of these core concepts.
Which subatomic particle determines the identity of an element?
Where are protons and neutrons located within an atom?
Understanding these basic parts of the atom is the first step. The behavior of the nucleus, especially when it's unstable, is the key to understanding radioactivity.